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LTE RRC Connection Setup Messaging

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LTE attach message sequence chart

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UE eNodeB

RACH: Random Access Preamble (RA-RNTI)

DL-SCH: RRC Connection Setup (SRBs, DRBs, UE Specific Configuration)

UL-SCH: RRC Connection Request (C-RNTI, M-TMSI, Establishment Cause)

UL-SCH: RRC Connection Complete (PLMN id, Dedicated NAS Info)

2

DL-SCH: Random Access Response (RA-RNTI, C-RNTI, Timing)

UE initiates a session to the eNodeB using a random RA-RNTI

eNodeB assigns a C-RNTI and adjusts timing

UE requests an RRC connection via the UL-SCH.

Network establishes the SRBs and DRBs based on the establishment cause.

UE acknowledges the setup of SRBs and DRBs.

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RACH: UE eNodeB: Random Access Preamble

Ran

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s P

ream

ble

Cyclic Prefix

Sequence

UE ID/RNTI Type RA-RNTI

UE ID/RNTI Value

• The terminal picks a preamble to send the random access message

• The preambles in LTE are defined from a Zadoff-Chu sequence

• The preamble consists of the cyclic prefix and a sequence

• The sequence identifies the UE that is initiating the random access

• The type of the UE and the UE ID value are included in the message

• RA-RNTI is used as a temporary identifier during the random access procedure

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DL-SCH: UE eNodeB: Random Access Response

Ran

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m A

cces

s R

esp

on

se UE ID/RNTI Type RA-RNTI

RA-RNTI Value

Timing Advance Value

Uplink Grant Resources

Temporary C-RNTI

• The eNodeB responds with a Random Access Response on the DL-SCH channel

• The UE is addressed with the RA-RNTI that was sent in the Random Access Preamble

• The message carries a Timing Advance that is used to adjust the UE transmitter timing

• This adjustment will synchronize the UE transmitter so that the transmissions from the UE are received within the receive timing window

• The message may carry an uplink resource assignment

• The message also assigns a C-RNTI that will be used to address the UE

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UL-SCH: UE eNodeB RRC Connection Request

RR

C C

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equ

est

UE ID/RNTI Type C-RNTI

UE ID/RNTI Value

UE-Identity S-TMSI

MMEC

M-TMSI Establishment

Cause mo-Signalling

• The UE has received the Random Access Response based on the RA-RNTI.

• The Random Access Response assigns a C-RNTI and resources for transmission of the RRC Connection Request

• The message identifies the UE with the C-RNTI

• The message contains the UE-Identity

• IMSI is sent in the message if this is the first attach to the network

• If the terminal had attached previously, the S-TMSI is included in the message

• The message also contains the establishment cause.

• In this example, the RRC Connection Request is sent with “Mobile Originated Signaling” cause.

• Note that the eNodeB may optionally send a contention resolution message on receipt of this message

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Esta

blis

hm

ent

Cau

se emergency

highPriorityAccess

mt-Access

mo-Signalling

mo-Data

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DL-SCH: UE eNodeB RRC Connection Setup

RR

C C

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nec

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n S

etu

p

UE ID/RNTI Type C-RNTI

UE ID/RNTI Value

Radio Resource Config Dedicated

SRBs to Add/Modify

SRB Identity

UL AM RLC

T Poll Retransmit

Poll PDU

Maximum Retrans. Threshold

DL AM RLC T Reordering

UL-SCH Config

MAX HARQ TX

Periodic BSR Timer

Retransmission BSR Timer

PHR Config

Periodic PHR Timer

Prohibit PHR Timer

DL Path Loss Change

Uplink Power Control Dedicated

P0 UE PUSCH

Delta MCS Enabled

P0 UE PUCCH

PSRS Offset

Filter Coefficient

• The message identifies the signaling radio bearer (SRB)

• The configuration parameters carried in the message are described in the next two slides

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RRC Connection Setup Configuration - 1

RLC Uplink Configuration

• Timer for status report polling

• Number of retransmissions of buffer status report

• Control plane retransmission limit

RLC Downlink Configuration

• The maximum time to wait for packet reordering

UL-SCH configuration

• Maximum number of Hybrid ARQ transmissions

• Periodic and regular buffer status report (BSR) timer

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RRC Connection Setup Configuration - 2

Power Headroom Report (PHR) Configuration

• Periodicity of the PHR

• Downlink Path Loss more than the specified value also triggers PHR (provided the Prohibit PHR timer has expired)

Uplink Power Control Parameters

• P0-UE-PUSCH and P0-UE-PCCH values are used to determine the nominal power of the uplink transmissions

• “pSRS Offset” determines the uplink Sounding Reference Signal power

• Delta MCS (choose between Ks = 0 and Ks = 1.25)

• Filter Coefficient value for RSRP (Reference Signal Received Power) measurement used to calculate path loss

• Reference: 36.213 clause 5.1.1.1

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UL-SCH: UE eNodeB RRC Connection Setup Complete

RR

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Co

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lete

Selected PLMN Identity

Registered MME (Optional)

PLMN Id

MMEGI

MMEC

Dedicated Info NAS Octets carrying NAS message

• UE sends this message on receipt of the RRC Connection Setup message

• “Dedicated Info NAS” is used to transfer UE specific NAS layer information between the network and the UE. The RRC layer is transparent for this information.

• The message may optionally contain registered MME

• The RRC Connection Setup Complete may also carry octets for a NAS message exchanged between the UE and the MME

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